The present invention relates to a cable joint between two relatively high voltage electric power cables, each cable having a conductor surrounded by, successively, a core insulation, a semi-conductive screen, a wired-screen and an outer cable sheath, said cable joint comprising, successively from an innermost to an outer side:
a junction connector adapted for connecting together bared ends of said conductors;
a semi-conductive layer adapted for covering said junction connector and ends of said core insulations, but separated from the semi-conductive screens of the two cables;
an insulating layer adapted for covering said semi-conductive layer and bared ends of said core insulations;
a conductive layer adapted for covering said insulating layer;
bared ends of conductive wires of said wired-screens; and
an insulating joint-cover adapted for covering said cable joint including ends of the outer cable sheaths of the two cables.
Such a cable joint is already known in the art, e.g. from the European Patent Application EP-A2-0 272 131 xe2x80x9cHV cablesxe2x80x9d of RAYCHEM GmbH, first filed on Dec. 12, 1986 (GB 8630335). Therein, a conductive layer is positioned on top of a stress-controlling layer so as to provide a Faraday cage around the connector. Additionally to this conductive layer forming an electromagnetic protection, the bared ends of the screen or earth wires are formed into a pigtail, brought together over the joint and interconnected by a crimp connector. The joint is further enclosed within an outermost heat shrinkable tube of insulating material forming the joint-cover recovering the outer jacket of each of the cables.
In such a known cable joint, the relatively large diameter of the screen wires arranged in pigtails and more particularly of the crimp connector interconnecting them is located over the joint connector. This provides locally a larger radius of the cable joint, i.e. a bump. As a consequence, when the heat shrinkable tube is applied onto the joint, it exerts thereon a pressure that is locally higher over the bump. The underlying layers are then over-compressed by the bump with the risk of damaging them and of also damaging the joint-cover, e.g. but cutting. The bump is even more detrimental in case of a joint containing mastic inside. Mastic is generally used in a cable-joint as insulating or filed controlling material and may for instance be applied below the insulating layer. A problem with a heat shrinkable tube is that the extra pressure caused by the bump would make the mastic to flow away.
A possible solution to avoid this problem is to locate the screen interconnection, i.e. the crimp connector, at an extremity of the joint. However, the screen wires arranged in a pigtail and running over the joint then still form a longitudinal bump. Additionally, the length of the whole joint will be increased, with all the drawbacks thereof: cost, accessibility, . . .
An object of the present invention is to provide a cable joint of the above known type but with a smooth wired-screen connection, while remaining relatively cheap and simple, and reducing as much as possible the length of the outer joint-cover.
According to the invention, this object is achieved due to the fact that the bared ends of said conductive wires are substantially equally distributed over the circumference of the underlying conductive layer, that a conductive metal plate adapted for covering the ends of said conductive wires is provided between said conductive layer and said joint-cover, and that said joint-cover is made of retracted cold shrinkable insulating material.
In this way, an asymmetrical connection is avoided because there is no longer a crimp connector lying on the joint. The diameter of the joint is maintained constant owing the distribution of the screen wires all over it. Moreover, a good electrical contact is ensured between the screen wires of the two cables via the conductive metal plate that is continuously pressed thereon by the shrinked or retracted cold shrinkable joint-cover. Furthermore, since the screen connection is located over the joint connector, the length of the joint, and more particularly of its outer joint-cover or protective sleeve remain relatively short.
It is to be noted that the joint-cover disclosed in the above-mentioned European Patent Application is a heat shrinkable joint-cover, which is thus positioned by heating. Once the heating is removed, the joint-cover remains in place but, contrarily to the present cold shrinkable joint-cover, a pressure is then no longer exerted on the underlying layers of the joint. The cold shrinkable joint-cover maintains a pressure on the underlying layers of the joint for ever and this maintains a good electric contact between the conductive metal plate and the underlying conductive wires. This permanent pressure is further cylindrically distributed in a symmetrical way so that the underlying insulating or field controlling mastic is equally spread over the joint. Another advantageous effect of this symmetrical pressure is that the mastic is prevented to flow away in any particular direction. This avoids the creation of gaps with a resulting risk of partial discharges and product failure.
It is also to be noted that the French Patent Application FR-2 551 927 xe2x80x9cDispositif de blindage xc3xa9lectromagnxc3xa9tique de jonctionxe2x80x9d of the SOCIETE DES ETABLISSEMENTS JACQUES DUBOIS and Ciexe2x80x94FR, filed on Sep. 8, 1983, also discloses a cable joint. However, this other known cable joint relates to low voltage cables adapted to transmit electronic signals such as telecommunication (PTT) signals. Although this known document discloses a sleeve of braided wires interconnecting the wired-screens of the cables, the purpose of the sleeve is to ensure the continuity of the electromagnetic shielding of the cables. The known sleeve is thus not adapted to allow relatively high currents to flow therethrough and the cables are further not provided with all the above mentioned layers. These cables are thus not adapted to withstand the above-mentioned relatively high voltage.
The two known documents relate to electromagnetic shielding of cables and not to screen interconnection as it is the subject of the present invention. However, applying the knowledge of this low voltage joint to the above known high voltage joint will not lead to the present invention. Indeed, none of the known documents teaches the use of a cold shrinkable joint-cover adapted to exert a permanent pressure onto the conductive metal plate for maintaining a good electric contact between this plate and the underlying conductive wires.
The European Patent EP-B1-0 549 942 entitled xe2x80x9cElectrical cable junction, premounted junction, assembly and manufacturing processxe2x80x9d of EUROMOLD (BE), first filed on Dec. 17, 1991 (FR 19910015666) also discloses a conductive metal plate that is a screen tap mounted on the bared ends of the semi-conductive screens of the cables. This known screen tap is of a relatively complex design and is thereby relatively expensive. It further requires a special tool to fix the screen tap onto the semi-conductive screen of the cable. Therefore, using the teaching of such a screen tap to apply it to the present improvement of a screen connection will result in making the latter complex and expensive and it thus against the object of the present invention.
The present invention also relates to a method of forming a joint between two relatively high voltage electric power cables.
Such a method is well known in the art and is for instance disclosed in the above mentioned European Patent Application EP-A2-0 272 131. Additionally to the known problems associated to a cable joint realized according to the known method and mentioned above, it is to be noted that the method disclosed in this European Patent Application further requires the use of a special crimping tool. This crimping tool is necessary to realize the interconnection of the bared ends of the screen or earth wires that have each to be arranged in a pigtail. The use of the crimping tool delays the creation of the joint and is not easy to manipulate in the field.
A further object of the present invention is to provide a method of forming a cable joint relatively faster as with the known method and with and improved or smoother wired-screen connection, while remaining relatively heap in production cost.
According to the invention, this further object is achieved due to the act that, in the present method:
a joint-body assembly comprising a cold shrinkable joint-body expanded on a first support tube, is slid over a first of said cables,
a joint-cover assembly comprising a cold shrinkable joint-cover expanded on a second support tube, is slid over the second of said cables,
xe2x80x83and, for each cable:
an outer cable sheath is removed to expose an underlying wired-screen,
conductive wires of said wired-screens are bend back over said outer cable sheath to expose an underlying semi-conductive screen,
said semi-conductive screen is removed to expose a core insulation, and
said core insulation is removed to expose a bared conductor,
xe2x80x83then:
said bared conductors are connected together by means of a junction connector,
said junction connector and, partially, said core insulations are covered with a semi-conductive layer, separated from said core insulation,- said joint-body assembly is slid over said semi-conductive layer and bared ends of said core insulations, and said first support tube is removed,
the shrinked joint-body is covered with a conductive layer,
said conductive wires are bend back over said conductive layer so as to cover substantially the whole surface of said conductive layer,
said conductive wires, distributed over the circumference of said conductive layer, are at least partially covered with a conductive metal plate, and
said joint-cover assembly is slid over said conductive metal plate so as to cover ends of the outer cable sheath of the two cables, and said second support tube is removed so that the cable joint, including the ends of the outer cable sheath of the two cables, is covered by the shrinked joint-body.
By distributing the conductive wires of the wired-screens, or screen wires, over the circumference or surface of the conductive layer and by covering them with a conductive plate, a smooth wired-screen connection is obtained. This leads to a symmetrical connection with a constant external diameter. Furthermore, a good electrical contact is ensured by the conductive metal plate that is continuously pressed on the screen wires of the two cables owing to the permanent pressure exerted by the retracted cold shrinkable joint-cover. This permanent pressure is further cylindrically distributed in a symmetrical way.
Because of the conductive metal plate is not covering more than the screen wires over the conductive layer, and is thus located over the joint, the length of the outer joint-cover or protective sleeve also remains relatively short.
Furthermore, no tool is required to bend back the screen wires, nor to place the conductive metal plate over these screen wires. This facilitates and increases the speed of creation of the joint in the field.
Another characterizing embodiment of the present method is that said cold shrinkable joint-body comprises a layer of insulating material and an inner layer of mastic.
In this way, and because the permanent pressure exerted by the retracted cold shrinkable joint-cover is cylindrically distributed in a symmetrical way, the underlying mastic is equally spread over the joint. As a result, the mastic, being insulating or field controlling, is prevented to flow away thus leaving, at particular locations, a gap with a resulting risk of partial discharges and product failure.
Another characterizing embodiment of the present method is that said conductive metal plate is rolled around the joint, covering the conductive wires of said wired-screens.
The metal plate is thus easily positioned and the sliding of the cold shrinkable joint-cover over this metal plate is facilitated.
Further characterizing embodiments of the present cable joint with the improved screen connection and of the method for obtaining such a cable joint are mentioned in the appended claims.